The benefits of root zone heating to help extend a plant's growing season or increase plant yield have been recognized and employed by gardeners for centuries (3). Growers have devised hundreds of methods to keep plant roots warm and looked for ways to heat the ambient air for plant foliage as well. The reasons for keeping roots and foliage warm might seem obvious, but the physiological processes occurring in the root zone are quite complex and affect the growth and development of plants profoundly.
Every physiological and developmental process in a plant has an optimum temperature for different species. For example, root initiations for semi-tropical plants are better at higher temperatures whereas temperature zone crops are better at lower temperatures. For example root development occurs when the soil temperature is 78° F. for poinsettias but roses only require root temperatures of 56° F. (4).
Research has shown that the optimum medium root zone temperature for propagating temperate plants ranges between 18-25 C (65 and 77 F) and 25-30 C (72-84 F) for warm/tropical climate species (5). Various studies evaluating the impact of changing root zone temperatures on crop growth have indicated that response depends on many factors including air temperature, light intensity and the species involved. Other researchers have reported that low temperatures in the root zone reduces stem elongation in Dendranthema plants (6), and flower formation in cucumber (7). Sandwell, (5) observed that root-zone heating increased tomato yield, especially under low night air temperatures of 9° C. Gosselin and Trudel, (8) found that maximum tomato yields were obtained at a night air and root-zone temperature combination of 18° C. and 24° C., respectively. Piore et al. (9) showed that when the root-zone temperature in Ricinus communis was decreased below a threshold value, leaf growth occurred preferentially at night and was strongly inhibited during the day.
Tomatoes are a widely used vegetable which are utilized fresh or as a processed product. Tomatoes require soil temperatures above 55 F and should be transplanted in the field when the soil temperatures reach 65° F. and daytime air temperatures average 55° to 60° F. The major impact of transplanting to cold soils is an inhibition of growth in general but root growth in particular due to low phosphorous (P) uptake. This is due mainly to the fact that cold soils slow the activity of microorganisms that place phosphorous (P) in solution thereby slowing diffusion to roots retarding root growth. Because root growth is critical to plant establishment a high phosphorous (P) containing starter solution is recommended.
For root zone temperature management one of the best arrangements for the plants growing inside of a greenhouse is to allow the plant container to rest directly on the concrete floor with nothing above it to hinder light penetration. One of the advantages of the pots being placed directly on the concrete floor is that the concrete will act as a thermal buffer with its “sensible heat” to keep the soil in the pot from cooling down during the night.
However, optimization of space and worker discomfort from stooping to the ground has led to most plants being elevated on benches or racks and therefore not resting on the concrete floor which would act as a temperature moderator at night to the soil in the pot. Greenhouse spacing is often limited and many ingenious methods to optimize the premium space have been developed. These methods include systems of racks, hanging baskets and moveable platforms known as roll-out benches (1). Therefore since fewer plants and their containers are not being placed directly on the concrete floor to take advantage of the thermal mass inertia of the concrete that is heated during the day a new viable approach is needed to keep the root system warm. Previous approaches to root zone heating have included heating underlayments. These underlayment systems involve electric heating pads placed under the pots of a system pumping a heated transfer fluid underneath the pots. These are heating systems that are costly to install and are not passive but require external fuel or electricity.
Phase change materials (PCMs) have been used in greenhouses, but only to control the whole greenhouse temperature, e.g., just like current greenhouse central heating and cooling systems. Levav and Zamir (11) reported that the use of PCMs in greenhouses achieved the required air temperature without increase in the relative humidity. However, the cost of the amount of PCM was too high. Kurklo (12) reported on a system that consisted of cans filled with PCM and hot daytime air was passed over them and then interior air was passed over the cans at night to maintain warmth. The air temperature was able to increase up to 15° F. One study that involved 13.5 tons of PCM in a greenhouse showed an 80% reduction in propane gas when compared to a similar neighboring greenhouse (13). In another design and experimental study the researchers showed a 40% reduction in heating load and a 30% energy savings (14). The conclusion is that PCMs can be used effectively in greenhouses for energy reduction. However, the amount of PCM is high if it is being used to thermally regulate the entire structure.
In most agricultural greenhouse operations labor is the largest expense with energy consumption being the second largest expense. In traditional greenhouse operations space heating composes 70% to 80% (see FIG. 1) of the total energy used (1) Greenhouses consume large amounts of energy for heating and cooling. A greenhouse heating load is the greatest at night. A central heating system for a greenhouse heats the whole greenhouse.
Many plant stems and leaves can tolerate cooler temperatures as long as their root system remains warmer. There are approaches that use underlayment heating systems to heat just the root system and not the entire greenhouse. These approaches involve an electrical resistant underlayment or heating a fluid and pumping it under the plants and pots in order to keep the roots warm and not warming the entire greenhouse.
It is possible to grow many summer plants in 45° F. ambient air as long as the root zone is in the 70 s. Root zone temperature is more critical than leaf temperature in achieving good plant growth. The biochemical processes that sustain a healthy plant are based on the root zone environment. As long as the leaf tissue does not freeze or fall below 40° F. for sustained periods and the root zone stays in the 65° to 80° F. range, you can grow many summer plants in an ambient temperature of 45° F.